Device for reading and/or writing optical recording media

ABSTRACT

The present invention relates to an appliance for reading from and/or writing to optical recording media  1  having a first laser diode LD1 for producing a first scanning beam AS1 at a first wavelength λ 1  and having a second laser diode LD2 for producing a second scanning beam AS2 at a second wavelength λ 2 , with the scanning beams AS1, AS2 running along a common optical axis  9 , scanning an information layer  6  on the recording medium  1  and falling on a single photodetector  8  in order to produce an information signal IS, with a beam combination element being arranged at one point on the optical axis  9 . According to the invention, the beam combination element is a diffraction grating  12.

[0001] The present invention relates to an appliance for reading fromand/or writing to optical recording media, which appliance uses scanningbeams at different wavelengths, which scanning beams run along a commonoptical axis, scan the recording medium, and are detected by a singlephotodetector. In particular, the appliance has an optical scanner forreplaying and recording data on optical rewritable disks.

[0002] Optical scanners, which can both replay and write to DigitalVersatile Disks (DVD) and Compact Disks (CD), require two differentlaser wavelengths. For this reason, CD-compatible DVD players andrecorders are fitted with two different laser diodes. This discreteconstruction results in increased financial costs due to theadditionally required optical components. Recently, so-called twin laserdiodes have been available as one approach to reduce the number ofcomponents required. These comprise two laser diodes at differentwavelengths, which are mounted laterally separated in a common laserhousing.

[0003] The lateral separation of the two light sources results in theemitted radiation through the optical system of the scanner being imagedat two mutually separate light spots in the information-carrying layerof the optical disk. This means that two laterally separate foci are inturn produced in the plane of the detector in which the light reflectedfrom the disk is imaged. The desired use of a common detector for bothwavelengths is thus impossible.

[0004] U.S. Pat. No. 6,043,911 discloses an appliance which usesscanning beams at two wavelengths, which are combined by means of a beamcombination element in order to propagate along a common optical axis.This known appliance has the disadvantage that the beam combinationelement, which consists of a combination of a prism and a hologram, iscostly to produce. Furthermore, the hologram is not optimally matched todifferent characteristics of the light sources that produce thedifferent wavelengths, and this leads to more or less severe disturbanceeffects.

[0005] One object of the present invention is to propose an improvedappliance. This object is achieved by the measures specified in theclaims.

[0006] According to the invention, the beam combination element is inthis case a diffraction grating. This has the advantage that itscharacteristics can be calculated, and can thus be optimally matched tothe characteristics of the light sources which, in particular, are laserdiodes. The characteristics of the diffraction grating are in this casepreferably calculated using one of the calculation methods specified inthe following text. The information layer is an information-carryinglayer on the recording medium which, for example, may be an optical disksuch as a CD, DVD, or else some other optical recording medium which caneither only be read from, can only be written to, or can be both readfrom and written to.

[0007] The diffraction grating preferably has grating lines with a blazeprofile or a profile which is similar to a blaze profile. In the case ofa blaze profile, a grating line does not have a rectangular crosssection, but an essentially obliquely running cross section. The profileis thus, for example, a sawtooth profile. One advantage of using a blazeprofile is that the diffraction efficiency is used optimally, and asgreat an intensity as possible is coupled from each of the light sourcesinto the combined beam path. This thus results in the minimum possibleoptical losses.

[0008] According to the invention, the grating lines are provided with astepped profile. This blaze-like profile has the advantage that it canbe produced with little effort but nevertheless has characteristicswhich are virtually just as good as a pure blaze profile.

[0009] The grating lines of the diffraction grating are preferablystraight and parallel, which is advantageous since they can be producedeasily. In many cases, this provides sufficiently good quality,particularly if the diffraction grating is arranged in the parallelbeam. If the diffraction grating is arranged in the divergent orconvergent beam, the grating lines are preferably designed such thatthey are curved. This has the advantage that, because the distancesbetween the grating lines thus differ as a function of the location, thediffraction requirements, which differ as a function of the location,are satisfied in the non-parallel beam, and aberrations are corrected.

[0010] Even when using curved grating lines, at least one grating lineis preferably straight. This has the advantage that the curvature of thegrating lines can be determined particularly easily, starting from astraight grating line. The distance between this grating line and theoptical axis preferably corresponds to half the distance between one ofthe light sources and the optical axis.

[0011] The invention provides for the diffraction grating to beoptimized to the respective first-order diffraction for bothwavelengths. Particularly when using the wavelength combination 650 nm,780 nm, it is in each case optimal to use first-order diffraction withrespect to the efficiency and simplicity of the grating architecture.Other combinations of diffraction orders are also worthwhile for otherwavelength combinations. This also includes not only the zero order butalso the second or higher orders.

[0012] In the simplest case, the laser diodes that produce light atdifferent wavelengths are arranged such that the scanning beams producedby them run parallel to one another and parallel to the optical axis.The invention provides for both laser diodes to be arranged tilted withrespect to the optical axis. This has the advantage that, in conjunctionwith the diffraction grating, this results in an intensity profile whichis as axially symmetrical as possible.

[0013] The diffraction grating is also preferably arranged rotated withrespect to the optical axis for this purpose. It is particularlyadvantageous for the laser diodes and the diffraction grating to bearranged rotated such that a zero-order virtual light source comes torest on the optical axis.

[0014] According to the invention, the diffraction grating isfurthermore oriented such that the side spots are oriented at rightangles to information tracks on the optical recording medium. Side spotsare focus points of secondary beams of a different order to those forwhich the diffraction grating is optimized. If required, thesediffraction orders are deliberately planned for determining thediffraction grating profile with a suitable intensity. The informationtrack is, for example, a spiral or circular track of elongatedinformation markings on a conventional optical disk. The alignment ofthe diffraction grating according to the invention has the advantagethat the side spots can be used to detect possible disk tilting or todetect any discrepancy between the scanning spot and the track centre,using known methods.

[0015] According to the invention, the laser diodes and the diffractiongrating are integrated in one module. This has the advantage that themodule is delivered as a prefabricated part, having been subjected toquality control, for installation, with fewer assembly and adjustmentsteps than being required during installation.

[0016] The diffraction grating is advantageously arranged in the beampath coming from the recording medium, but still upstream front of thephotodetector. This has the advantage that the beam combination takesplace only in the rearward path of the scanning beam. The diffractiongrating can thus be designed to be simpler since any errors which may becaused by it have scarcely any effect in the remaining short beam path.

[0017] In this case, the diffraction grating and the detector elementare advantageously integrated in one module, too.

[0018] A further diffraction grating is preferably arranged in the beampath. This has the advantage of producing further secondary beams whichare used, for example, for tracking. If the further diffraction gratingis a Ronchi grating, then this has the advantage that secondary beamsare produced for only one of the wavelengths. This is particularlyadvantageous when further secondary beams, for example for carrying outthe known three-beam tracking method, are intended to be used in anycase for only one of the wavelengths.

[0019] The invention provides for only one of the two laser diodes to beoperated in each case for reading from a recording medium, while bothlaser diodes are operated simultaneously in order to record informationon the recording medium. The diffraction grating according to theinvention ensures that the spots of both laser diodes are superimposedon the optical recording medium, so that the energy which is requiredfor recording or deletion of data is advantageously applied by bothlaser diodes producing scanning beams simultaneously. Only one scanningbeam is required in each case for reading from the recording medium.Different wavelengths are preferably provided for writing or deletion,although it is within the scope of the invention for the same wavelengthto be used here.

[0020] One method according to the invention for producing a diffractiongrating, in particular for use in an appliance according to theinvention, is for the grating structure and the grating line profile tobe defined, for a corresponding height profile to be determined fromthis and to be subdivided into a staircase profile, and for the areas ofdifferent height produced in this process to be transferred to a blankby means of lithography and an etching process.

[0021] Further advantageous refinements of the invention are containedin the following description of exemplary embodiments. In the figures:

[0022]FIG. 1: shows a beam path using two laser diodes;

[0023]FIG. 2: shows a beam combination by means of a Wollaston prism;

[0024]FIG. 3: shows a beam combination by means of a diffractiongrating;

[0025]FIG. 4: shows a line profile of a diffraction grating;

[0026]FIG. 5: shows a beam path using a diffraction grating;

[0027]FIG. 6: shows a line structure of a diffraction grating accordingto the invention;

[0028]FIG. 7: shows a beam path for laser diodes arranged offset withrespect to the optical axis;

[0029]FIG. 8: shows an appliance according to the invention having adiffraction grating in the divergent beam;

[0030]FIG. 9: shows an appliance according to the invention having adiffraction grating in the parallel beam;

[0031]FIG. 10: shows an appliance according to the invention with adiffraction grating in the verification path;

[0032]FIG. 11: shows a table of calculated diffraction efficiency.

[0033]FIG. 1 shows the beam path in an optical scanner 3 of anappliances for reading from and/or writing to optical recording media 1,which has two laser diodes LD1, LD2. A so-called twin laser diode ordual laser diode comprises an arrangement of two separate laser diodesLD1, LD2, which are integrated in a common housing 2. For applicationsin the field of optical scanners 3 for reading from and writing tooptical recording media 1, a first laser diode LD1 emits at a firstwavelength λ₁=650 nm, and a second laser diode LD2 emits at a secondwavelength λ₂=780 nm. The radiation at the second wavelength λ₂ is usedin the illustrated case for reading from and writing to the older CDformat, while the first wavelength λ₁ is used for the newer DVD formats.As a consequence of the various requirements for the different diskformats, all the components of the scanner 3 must be optimized for bothwavelengths λ₁, λ₂. Thus, for example, the collimator lens 4 should haveas little dispersion as possible and, furthermore, the objective lens 5should compensate for the spherical aberration of the substratethicknesses sd₁, sd₂, which are different for CDs and DVDs. Therecording medium 1 in FIG. 1(a) is alternatively indicated with asubstrate thickness sd₁ for DVDs and with a substrate thickness sd₂ forCDs. The beam paths for the different laser diodes LD1, LD2 areillustrated in separate figures, FIG. 1(a) and FIG. 1(b), for the sakeof clarity. Appliances such as these are subject to the followingproblem: in principle, the optical scanner 3 is a diffraction-limitedimage of the laser source LD1, LD2 on the optical storage disk 1. In thecase of a twin laser diode, both laser sources LD1, LD2 are separatedlaterally in the mounting housing 2. They produce two scanning beamsAS1, AS2, which run along the optical axis 9 of the scanner 3. They passthrough a beam splitter 16 and are imaged by the optical systemcomprising the collimator lens 4 and the objective lens 5 at twomutually separate spots SP1, SP2 on the information-carrying layer 6 onthe optical disk 1. These two spots SP1, SP2 may in turn be regarded aslight sources, which are imaged via the objective lens 5 and thecylindrical lens 7 in the plane of the detector 8. The detector 8 isillustrated tilted through 90° in FIG. 1c and, in the illustrated case,has four quadrants A, B, C, D, which emit a respective electrical signalA1, B1, C1, D1. These signals are converted in a known manner, whichwill not be explained in any more detail here, by an evaluation unit 10to one or more information signals IS. As a consequence of theastigmatism which is introduced by the cylindrical lens 7, the imagesSB1, SB2 of the two light spots SP1, SP2 in the plane of the detectorare no longer of a diffraction-limited extent, but of a size which iscritically dependent on the focal length of the cylindrical lens 7. Theoriginal separation of the two spots SP1, SP2 is thus no longer ensuredin their images SB1, SB2. The problem will be illustrated by a numericalexample: the two laser sources LD1, LD2 are typically separated from oneanother laterally by about ld=0.1 mm. In the detector plane, this leadsto images SB1, SB2 which are likewise separated from one another byabout ld′=0.1 mm, depending on the focal length of the cylindrical lens7. The cylindrical lens 7 itself is normally chosen such that thislikewise results in an image SB1, SB2 of a spot SP1, SP2 on the detector8 likewise having a diameter db of about db=0.1 mm. Thus, in practice,two mutually shifted spot images SB1, SB2, each having four quadrants A,B, C, D, can be found, whose separation corresponds approximately totheir diameter db. One possible solution would be to use a detectorpattern formed from two photodiodes each having four quadrants. However,this solution option is expensive to implement in practice, since thedistance between the two spot images SB1, SB2 in the plane of thedetector 8 varies during adjustment of the cylindrical lens 7. The fixedimage separation as predetermined by a detector pattern can thus not bemaintained during construction and adjustment of the optical scanner 3.

[0034] An optical arrangement is desirable which allows the use of asingle detector 8 having four quadrants. The two spot images SB1, SB2should thus meet at the same position in the detector plane. Inprinciple, it is possible to distinguish between two different solutionapproaches for achieving this aim: firstly, the production of two spotsSP1, SP2 on the disk 1, which are located laterally at the sameposition. This results in the two spot images SB1, SB2 being concentricin the detector plane. Secondly, the imaging of the spots SP1, SP2,which are laterally separated on the disk 1, at the same position in thedetector plane.

[0035] For the latter, it is proposed that the birefringentcharacteristic of a Wollaston prism 11 be made use of in the detectorpath. This is illustrated in FIG. 2. For simplicity, the illustrationshows only those scanning beams AS1, AS2 which run backwards from therecording medium 1 in the direction of the detector 8. While thescanning beam AS1 at the wavelength λ₁ strikes the Wollaston prism 11 asan ordinary beam, and leaves it again without being refracted, thescanning beam AS2 at the wavelength λ₂ is refracted, as an extraordinarybeam. The arrangement is now chosen such that the images SB1, SB2 at thetwo wavelengths λ₁, λ₂ strike the detector 8 at the same lateralposition. The choice of a Wollaston prism 11 for beam combinationinvolves the following disadvantages: the polarization vectors of thetwo wavelengths must be at right angles to one another. This restrictsthe degree of freedom for variable orientation of the two laser diodesLD1, LD2. The production techniques mean that the two laser diodes LD1,LD2 can virtually never be aligned exactly with mutually perpendicularpolarization in the housing. This makes adjustment considerably moredifficult. The birefringence of the substrate of the recording medium 1rotates the polarization as a function of the position of the opticalscanner relative to the recording medium 1, particularly when usingquarter-wave phase plates, which are normally used for writing drivesbut are not illustrated here.

[0036] The abovementioned disadvantages make it clear that beamcombination using the polarization-dependent characteristics ofmaterials is not desirable. A method according to the invention, whichallows polarization-independent beam combination based on the twosolution approaches mentioned above will be described in the followingtext.

[0037] The basic principle of the invention as illustrated in FIG. 3makes use, in the inverse sense, of the dispersive characteristic of adiffraction grating 12. While in the case of a grating spectrometer, thepolychromic radiation which strikes the grating at a fixed incidenceangle α leaves it again in accordance with the grating equation

n*λ=d*(sin(α)−sin(β))   (1)

[0038] at different angles β in der n-th diffraction order, theradiation at the different wavelengths λ₁ and λ₂ in this case strikesthe diffraction grating 12 at the angles α₁ and α₂. The grating period dis chosen such that it results in identical exit angles β₁ and β₂ inaccordance with the grating equation. Thus, for β₁=β₂=0: $\begin{matrix}\begin{matrix}{\quad {{{{n_{1}*\frac{\lambda_{1}}{d}} - {\sin \left( \alpha_{1} \right)}} = 0};}} & {{{{n_{2}*\frac{\lambda_{2}}{d}} - {\sin \left( \alpha_{2} \right)}} = 0},}\end{matrix} & (2)\end{matrix}$

[0039] where n₁ and n₂ describe the diffraction orders used for beamcombination. These are chosen to be independent of one another from thestart. For example, it is possible to choose n₁=0 and n₂=1, which, withα₁=0, results in the required grating period becoming: $\begin{matrix}{d = \frac{1}{\lambda_{2} + {\sin \left( \alpha_{2} \right)}}} & (3)\end{matrix}$

[0040] Corresponding solutions of equation (2) are obtained for othercombinations of the diffraction orders n₁ and n₂. The radiation emittedat the wavelengths λ₁ and λ₂ from the two laser diodes strikes thediffraction grating at the angles α₁ and α₂ with respect to the normalto the surface. The figure shows the exit angle β which, according tothe invention, is chosen to be the same for both wavelengths.

[0041]FIG. 4 shows optimization according to the invention of the lineprofile of the diffraction grating 12. A blaze profile is used for thispurpose in FIG. 4(a), while a stepped profile is used as anapproximation of the blaze profile in FIG. 4(b). The stepped profile isin this case represented by 4 height steps. The equidistant steps h₁,h₂, h₃ are chosen such that, on average, they correspond to the blazeangle θ_(B) as shown in FIG. 4(a).

[0042] The suitable choice of the diffraction orders n₁ and n₂ is made,according to the invention, by-additionally taking into account thediffraction efficiency ε. The diffraction efficiency ε determines whichcomponents of the emitted laser light at the wavelengths λ₁, λ₂ isavailable for the optical system of the scanner 3. In principle, thediffraction efficiency ε is not only dependent on the choice of thediffraction order n₁, n₂, but is critically dependent on the structurefactor of the diffraction grating 12, that is to say on the profile ofthe individual grating lines 13. FIG. 4 illustrates examples of such aprofile of the grating lines 13. The asymmetrically formed blaze profileillustrated in FIG. 4(a) is, according to the invention, particularlysuitable for concentrating as large a proportion as possible of thediffractive radiation in only one diffraction order n. If the blazecondition $\begin{matrix}{{2\pi*\frac{x}{d}} = {\left. {\frac{2\pi}{n*\lambda}*\left( {n_{r} - 1} \right)*{h(x)}}\rightarrow{h(x)} \right. = {\frac{n*\lambda}{d}*\frac{x}{n_{r} - 1}}}} & (4)\end{matrix}$

[0043] is satisfied exactly for such a diffraction grating 12 whosesubstrate is characterized by the diffractive index n_(r), then adiffraction efficiency of ε=1 is obtained for the corresponding order n,and ε=0 for all other orders. In fact, it is evident from FIG. 4(a) andequation (4) that the blaze condition cannot be satisfied simultaneouslyfor both wavelengths λ₁ and λ₂. The diffraction efficiency ε is given by

ε(n)=|α(n)|²,   (5)

[0044] where α(n) describes the complex amplitude diffaction efficiency.The variable α(n) can be calculated for grating periods that are notexcessively small by: $\begin{matrix}{{a(n)} = {\frac{1}{d}{\int_{0}^{d}{{\exp \left( {{\varphi}(x)} \right)}*{\exp \left( {{- {2\pi}}\quad n\frac{x}{d}} \right)}{x}}}}} & (6)\end{matrix}$

[0045] where Φ(x) describes the relative phase of a beam that strikesthe grating step at the point x. For a blaze grating: $\begin{matrix}{{{\varphi (x)} = {2\pi \frac{n_{r} - 1}{n*\lambda}{h(x)}}},} & (7)\end{matrix}$

[0046] where h(x) represents the height profile of a grating line 13.The relationship with the blaze angle shown in FIG. 4(a) is given by:

h(x)=tan(θ_(B))*x for x ε[0,d]  (8)

[0047] If the grating profile is optimized for the wavelength λ₁ in thediffraction order n₁, then this results, for the wavelength λ₂ in theorder n₂, in: $\begin{matrix}{{ɛ\left( n_{2} \right)} = \frac{2 - {2*{\cos \left( {2\pi*\left( {{n_{1}*\frac{\lambda_{1}}{\lambda_{2}}} - n_{2}} \right)} \right)}}}{\left( {2\pi} \right)^{2}*\left( {{n_{1}*\frac{\lambda_{1}}{\lambda_{2}}} - n_{2}} \right)^{2}}} & (9)\end{matrix}$

[0048] on the assumption that the dispersion of the grating subtrate,that is to say the change in n_(r) with wavelength, is negligible.

[0049] It has been found that, in order to achieve a maximum light yieldfor both wavelengths for a given ratio of λ₁/λ₂=0.833, it isparticularly advantageous to choose the diffraction orders to ben₁=n₂=±1. As an example, FIG. 11 shows the diffraction efficiency ε forthe various diffraction orders n, assuming a grating profile with whichthe blaze condition is satisfied exactly for the wavelength λ₁=650 nm inthe first order. This blaze condition is correspondingly contravened forthe wavelength λ₂=780 nm. However, owing to the relatively smalldifference between the wavelengths λ₁, λ₂, it is evident that adiffraction efficiency ε of more than 90% can also be achieved for λ₂.In addition, FIG. 11 lists the diffraction efficiencies which resultwith the grating profile, in this case in the form of a 4-step profile,as sketched in FIG. 4(b) and formed from four discrete steps. A profilesuch as this can be produced more easily by lithographic exposure andsubsequent etching processes than the ideal blaze profile, which needsto be manufactured mechanically. The step heights h₁, h₂, h₃ and thenumber of steps are in this case chosen to achieve the best possibleapproximation to the optimized blaze profile. The diffractionefficiencies indicated in FIG. 11 for this profile were determined bynumerical evaluation of equation (5). In this case as well, it was foundthat a yield of more than 70% can be achieved for both wavelengths λ₁,λ₂.

[0050] Specific arrangements in optical scanners for beam combinationwill be described in the following text, in particular for diffractionorders n₁=n₂=±1.

[0051]FIG. 5 shows the beam path of an appliance according to theinvention for beam combination in the divergent scanning beam AS1, AS2.The radiation which is emitted in a divergent manner from the two laserdiodes LD1 and LD2 has no aberrations after diffraction at thediffraction grating 12 and propagates as if both wavelengths originatedfrom the same point, the virtual source VS. The two laser diodes LD1 andLD2 are in this case regarded as point light sources. After diffractionat the diffraction grating 12, the radiation from the two light sourcesLD1, LD2 propagates as if they originated in a single source, theso-called virtual source VS. With regard to the virtual source VS, thetwo actual sources LD1, LD2 are located at the lateral coordinate points(0,y₁) and (0,y₂), respectively. The diffraction grating 12 is locatedat a longitudinal distance Z₀. The beams which strike the diffractiongrating 12 centrally, that is to say at the coordinate (Z₀, 0) run alongthe optical axis 9 after diffraction, and the exit angle β for bothbeams AS1, AS2 is zero. The incidence angles α₁ and α₂ can be determinedon the one hand from the geometry of the arrangement to be:

y ₁ =Z ₀*tan(α₁); y ₂ =Z ₀*tan(α₂)   (10)

[0052] On the other hand, they must satisfy the grating equation (1) forβ=0, so that this results in the condition: $\begin{matrix}{{\Delta \quad y} = {{y_{2} - y_{1}} = {{Z_{o}*\left( {{\tan \left( {{arc}\quad \sin \frac{\lambda_{2}}{d}} \right)} - {\tan \left( {{arc}\quad \sin \frac{\lambda_{1}}{d}} \right)}} \right)} \approx {\frac{Z_{0}}{d}\left( {\lambda_{2} - \lambda_{1}} \right)}}}} & (11)\end{matrix}$

[0053] consigned with the last step in equation (11) being applicableand at the limit where λ_(1,2)<<d. Equation (11) allows the gratingconstant d, as required for recombination, and the position y₁ to bedetermined for a given distance Δy between the laser diode LD1 and thelaser diode LD2. For example, a grating constant of d=13 μm and aposition y₁=0.5 mm are obtained for Z₀=10 mm and Δy=0.1 mm.

[0054] If a simple linear diffraction grating 12 with a periodcorresponding to equation (11) is used, the combination of the twoscanning beams AS1, AS2 is ensured, but there is no diffraction-limitedspot SP1, SP2 on the information-carrying layer 6 of the recordingmedium 1. This is due to aberrations which occur during diffraction ofthe divergent beam AS1, AS2 on the linear grating. In order to preventthis, the diffraction grating 12 according to the invention isstructured to be far more complex than a simple diffraction grating.

[0055]FIG. 6 shows the line structure of an optimized diffractiongrating 12 with curved grating lines 13. As can be seen, one gratingline 13′ is not curved. The line separation d is in this case describedin cartesian form as a function of the coordinates (x,y) by d_(x)(x,y)and d_(y)(x,y).

[0056] The correct structure of the diffraction grating 12 isdetermined, as described in the following text, for the radiation fromthe laser diode LD1. Perfect correction for all aberrations is possibleonly for the wavelength λ₁, λ₂ of one of the two laser diodes LD1, LD2.The finite aberrations in the radiation from the laser diode LD2 arenegligible, as shown by numerical simulation calculations.

[0057] Each beam which originates from the laser diode LD1, that is tosay from the point (0,y₁) and which strikes the diffraction grating 12at the point (x_(a),y_(a)) is intended to be diffracted such that theresultant beam corresponds to that which is produced at the virtualsource VS at the point (0,0) and passes through the point (x_(a),y_(a))without being diffracted. This means that the exit direction of thediffracted beam is equal to the incidence direction of the virtual beam.In order to allow the diffraction on curved grating lines 13 to bedescribed correctly, the grating period, as illustrated in FIG. 6, isbroken down into the Cartesian coordinates d_(x) and d_(y) for eachgrating coordinate (x,y). The incidence angle α is subdivided in acorresponding way into its components a_(x) and a_(y).

[0058] Thus, for a beam from (0,y₁) to (x_(a),y_(a)) at the distance Z₀:$\begin{matrix}{{{\alpha_{x}\left( {a_{a},y_{a}} \right)} = {{arc}\quad {\tan \left( \frac{x_{a}}{\sqrt{Z_{0}^{2} + \left( {y_{a} - y_{1}} \right)^{2}}} \right)}}}{{a_{y}\left( {x_{a},y_{a}} \right)} = {{arc}\quad {\tan \left( \frac{y_{a} - y_{1}}{\sqrt{Z_{0}^{2} + x_{a}^{2}}} \right)}}}} & (12)\end{matrix}$

[0059] The virtual beam from the virtual source VS governs the nominalexit angle β, which is likewise represented in the form of components.This is equal to the incidence angle of the virtual beam, such that:$\begin{matrix}{{{\beta_{x}\left( {x_{a},y_{a}} \right)} = {{arc}\quad {\tan \left( \frac{x_{a}}{\sqrt{Z_{0}^{2} + y_{a}^{2}}} \right)}}}{{\beta_{y}\left( {x_{a},y_{a}} \right)} = {{arc}\quad {\tan \left( \frac{y_{a}}{\sqrt{Z_{0}^{2} + x_{a}^{2}}} \right)}}}} & (13)\end{matrix}$

[0060] The grating periods d_(x)(x_(a),y_(a)) and d_(y)(x_(a),y_(a)) arechosen such that the grating equation (1) with the angles α_(x), α_(y),β_(x), β_(y) calculated above is satisfied at each point (x_(a),y_(a))for the first diffraction order. Thus: $\begin{matrix}{{{d_{x}\left( {x_{a},y_{a}} \right)} = \frac{\lambda_{1}}{{\sin \left( {{arc}\quad {\tan \left( \frac{y_{a} - y_{1}}{\sqrt{Z_{0}^{2} + x_{a}^{2}}} \right)}} \right)} - {\sin \left( {{arc}\quad {\tan \left( \frac{y_{a}}{\sqrt{Z_{0}^{2} + x_{a}^{2}}} \right)}} \right)}}}{d_{x}\left( {x_{a},{y_{a} = \frac{\lambda_{1}}{{\sin \left( {{arc}\quad {\tan \left( \frac{x_{a}}{\sqrt{Z_{0}^{2} + \left( {y_{a} - y_{1}} \right)^{2}}} \right)}} \right)} - {\sin \left( {{arc}\quad {\tan \left( \frac{x_{a}}{\sqrt{Z_{0}^{2}} + y_{a}^{2}} \right)}} \right)}}}} \right.}} & (14)\end{matrix}$

[0061] The diffraction grating is completely characterized by equation14 and can be subdivided into individual grating lines 13. According tothe invention, the grating line 13′ at y_(a)=y₁/2 is a suitable point oforigin for the line structure. In this case, the denominator of d_(x)has a singularity, which means that the grating line 13′ runs parallelto the x axis. The profile of all the other grating lines 13 can becalculated by successive addition of d_(y). The structure of thediffraction grating 12 as illustrated in FIG. 6 correspondsqualitatively to the structure calculated in this way.

[0062]FIG. 7 shows the beam path for for laser diodes LD1, LD2 which arearranged offset with respect to the optical axis 9. The bold arrows inthis case indicate the emission angles of the intensity maxima. The twolaser diodes LD1 and LD2 are normally arranged such that the angledistribution of the intensity profile is aligned parallel to the Z axis.The lateral distance Δy(Z₁) between the intensity maxima depends on thedistance y₂(0)−y₁(0) between the laser diodes LD1, LD2 and thepropagation distance Z₁−Z₀.

[0063] The description so far has ignored the emission characteristic ofthe laser diodes. The laser diodes LD1 and LD2 have been assumed to bepoint light sources whose radiation has no specific angle distribution.FIG. 7 shows the direction in which the intensity maxima finally move,or how their lateral separation Δy(Z₁) increases in the longitudinaldirection. Thus, for this separation: $\begin{matrix}{{\Delta \quad {y\left( Z_{1} \right)}} = {\frac{{y_{2}(0)} - {y_{1}(0)}}{Z_{0}}*Z_{1}}} & (15)\end{matrix}$

[0064] If a collimator lens 4 is located at Z₁, then the distance Δy(Z₁)remains constant for Z>Z₁. y₂−y₁=0.1 mm, Z₀=10 mm and Z₁=20 mm will onceagain be assumed as a numerical example. This thus results in theemission maxima being separated by 0.2 mm. This value is small incomparison to the typical aperture diameter of an objective lens 5 ofabout 3-4 mm. This means that, in practice, the separation between theintensity maxima is scarcely evident. In contrast, the shift in theemission maxima with respect to the axis 9′ that is shown is morecritical. This shift y₁(Z₁) will assume approximately five times thevalue of Δy(Z₁) . For the above numerical example, this means that theintensity maximum is shifted through about 1 mm with respect to theoptical axis 9′. This shift thus amounts to approximately ¼ of the lensaperture, and should thus be corrected. According to the invention, thisis done as follows: the wave fronts that emerge from the diffractiongrating 12 correspond to spherical waves which originate from anemission point at the virtual source VS. It is thus possible to rotatethe subsequent optical system at any desired angles about the point VSwithout the previous steps for definition of the diffraction grating 12becoming invalid. There is thus no change in the characteristics of thecombined beams AS1, AS2, either, unless the characteristics of thediffraction grating 12 are changed. The rotation to be carried out iseffectively provided at an angle such that the intensity maximum of thelaser diode LD1 lies on the optical axis 9 of the subsequent system.FIG. 8 shows a correspondingly optimized overall system.

[0065]FIG. 8 shows the overall structure of the optical scanner 8 withthe diffraction grating 12 in the divergent beam. In order to keep theshift of the intensity maxima of the two laser diodes LD1, LD2 withrespect to the optical axis 9 of the optical system following thediffraction grating 12 as small as possible, the unit comprising thelaser diodes LD1, LD2 and the diffraction grating 12 is rotated about anaxis which is at right angles to the plane of the drawing and runsthrough the virtual source VS. The optimum rotation angle is the meanvalue of the emission angles of the two laser diodes LD1, LD2 afterpassing through the diffraction grating 12.

[0066] According to the invention, when using a corresponding smalldistance Z₀, a module 14 is provided as an integrated overall component,comprising a twin laser diode LD1, LD2 and a diffraction grating 12. Theorientation of the optical system relative to the diffraction grating 12is chosen such that the remaining diffraction orders of the diffractiongrating 12 result in light spots which are oriented at right angles tothe tracks on the optical disk 1. When using additional detectorelements, which are not illustrated here, these side spots are used todetect radial tilting of the optical disc. A further grating 14 isshown, as an option. This is used to detect any tracking error duringreproduction of a CD disk, based on the known three-beam trackingmethod. Its grating lines are for this purpose oriented approximately atright angles to those of the diffraction grating 12, so that the sidespots which result from the further grating 15 on the disk 1 are alignedalong the tracks. Since the further grating 15 is not required forreading from DVD disks, the invention provides for such a grating to beused in which no diffraction occurs for the wavelength λ₁. This is thecase, for example, for a Ronchi grating with ΔΦ=π for 650 nm.

[0067] The beam combination in the collimated beam is simpler than inthe case described above. In this case, as is illustrated in FIG. 9, theradiation AS1, AS2 which emerges in a divergent form from the laserdiodes LD1 and LD2 is first of all collimated by means of an appropriatecollimator lens 4. Owing to the different object positions of the laserdiodes LD1 and LD2, the collimated beam comprising the wavelengths λ₁,λ₂ has different field angles. These are matched to one another by thediffraction grating 12 according to the invention, which is located inthe collimated beam. The diffraction of a collimated beam from a lineargrating does not result in any aberrations, and the diffraction grating12 is advantageously in the form of a simple diffraction grating. Inorder to achieve a maximum photon yield for both wavelengths λ₁, λ₂,both laser fields are diffracted in the respective first order, asalready described. The grating period is designed taking into accountthe difference between the field angles Δα=α₂−α₁ of the collimatedbeams, such that: $\begin{matrix}{{2*{\tan \left( \frac{\Delta \quad \alpha}{2} \right)}} = {\left. \frac{\Delta \quad y}{f_{coll}}\rightarrow{\Delta \quad \alpha} \right. = {2*{\arctan \left( \frac{\Delta \quad y}{2*f_{coll}} \right)}}}} & (16)\end{matrix}$

[0068] where f_(coll) describes the focal length of the collimator lens4. Subject to the condition β=0, equation (2) is used to uniquely definethe grating periode d and the incidence angle α₁: $\begin{matrix}{d = {\frac{\lambda_{1}}{\sin \left( \alpha_{1} \right)} = \frac{\lambda_{2}}{\sin \left( {\alpha_{1} + {\Delta \quad \alpha}} \right)}}} & (17)\end{matrix}$

[0069] Numerical example: for a collimator focal length of f_(coll)=20mm and a lateral separation Δy=0.1 mm, this results in Δα=0.286°. Usingequation (17), the angle and grating period can be defined asα₁=1.43°and d=26 μm, respectively.

[0070]FIG. 10 shows the beam combination in the backward path of anappliance according to the invention. The diffraction grating 12 is inthis case arranged in the verification path of the opticals scanner 8.This arrangement corresponds to that illustrated in FIG. 2, with adiffraction grating 12 according to the invention being used for beamcombinations, instead of the Wollaston prism 11. In this case, theradiation emitted from the two laser diodes LD1 and LD2 is initially notcombined so that, as is shown in FIG. 1, two laterally separate spotsSP1, SP2 are produced on the information layer 6 on the optical disk 1.The boundary condition that both spots SP1, SP2 are imaged at the sameposition SB1, SB2 in the detector plane is in this case satisfied by thediffraction grating 12 in the backward path. The structure of thediffraction grating 12 may in this case be in the form of a simplelinear grating, even if it is located in the non-parallel beam path. Theaberrations which result from the diffraction on the linear grating arenegligible at this point, in contrast to the structure described inconjunction with FIGS. 5-8. For photon yield reasons, the diffractiongrating 12 is once again used for both wavelengths λ₁, λ₂ in the firstdiffraction order. A module 14′, which contains the diffraction grating12 and the detector 8, is illustrated by dashed lines, as analternative.

[0071] Additional application options for the invention are specified inthe following text. The first two options for beam combination result intwo overlapping light spots SB1, SB2 being produced on the optical disk1. During normal operation of an optical scanner 8, the successive useof the two wavelengths λ₁, λ₂ is preferred, that is to say the laserdiode LD1 at the wavelength λ=650 nm for DVDs, and the laser diode LD2at the wavelength λ=780 nm for CDs, with the method according to theinvention opening at novel applications for optical data storage. Theseinclude, for example, the so-called two-photon processes. Theseprocesses make use of a memory layer 6 on the disk 1, which usesmolecular electronic transitions for writing an information unit. Inthis case, the molecular transition from a state ZA to another state ZBtakes place via an intermediate level ZC. Light from the laser diode LD1is used, by way of example, to stimulate the transition ZA→ZC, while thetransition ZC→ZB is stimulated by the radiation from the laser diodeLD2. In contrast, the information that has been written is read usingonly one of the two laser diodes LD1, LD2. The use of such two-photonprocesses will make it possible to achieve greater data reliability inthe future. So-called “pre-heat” processes are envisaged as a furthernovel application of the overlapping spots SP1, SP2. In this case, byway of example, the light from the laser diode LD2 ensures that a largearea of the memory layers 6 on the optical disk is heated up, while theinformation is written to the memory layer 6 only by appropriate pulsesof the laser diode LD1. This likewise has advantages over the presentday methods in terms of better data reliability with regard to deletionprocesses and the higher performance densities that can be achieved.Higher performance densities are desirable, for example, in the case ofoptical storage disks 1 with a number of information-carrying layers 6.

[0072] The invention relates to a diffractive method for forming theradiation emitted from the two laser diodes LD1, LD2, so that it ispossible to use a single detector 8. On the one hand, a way is indicatedto achieve two colinearly arranged foci SP1, SP2 on the optical disk 1,so that a single detector 8 can be used. On the other hand, a way isdescribed for imaging the light spots SP1, SP2, which are separated onthe optical disk 1, on a common detector 8. The use of a Wollaston prism11 has the following disadvantages: the polarization of the laser diodesLD1, LD2 cannot be chosen freely. It cannot be used in the forward path,and laterally separate spots SP1, SP2 occur on the disk 1. Wollastonprisms 11 are comparatively expensive optical components, since theycannot be manufactured from plastic. The use of the diffraction grating12 makes it possible, according to the invention, for the radiationwhich is emitted from two laterally separated monochromatic lightsources, in this case laser diodes LD1, LD2, to be-formed such that thelight beams at the two wavelengths λ₁, λ₂ have a common axis 9 afterpassing through the diffraction grating 12. This makes it possible toachieve a simple concept for an optical scanner 8 for replaying and forrecording DVDs and CDs. The dispersive characteristic of the diffractionin the grating is used for combination of the radiation, with the firstdiffraction order n=±1 being used for both wavelengths λ₁, λ₂. A complexline structure for correction aberrations is described for use in theforward path of the scanner 8. In order to achieve a diffractionefficiency that it as high as possible for both wavelengths λ₁, λ₂, thatis to say in order to achieve low light losses, a blaze geometry indiscrete form is used for the step shape of the diffraction grating 12.Pre-heat recording and two-phonton processes are mentioned as furtherpossible applications of the method.

1) appliance for reading from and/or writing to optical recording media(1) having a first laser diode (LD1) for producing a first scanning beam(AS1) at a first wavelength (λ₁) and having a second laser diode (LD2)for producing a second scanning beam (AS2) at a second wavelength (λ₂),with the scanning beams (AS1, AS2) running along a common optical axis(9), scanning an information layer (6) on the recording medium (1) andfalling on a single photodetector (8) in order to produce an informationsignal (IS), with a beam combination element being arranged at one pointon the optical axis (9), characterized in that the beam combinationelement is a diffraction grating (12) which is optimized for higher thanzero-order diffraction for both wavelengths (λ₁, λ₂). 2) applianceaccording to claim 1, characterized in that the diffraction grating (12)has grating lines (13) with a blaze profile, or with a profile which issimilar to this: 3) Appliance according to claim 2, characterized inthat grating lines (13) with a stepped profile (h₁, h₂, h₃) areprovided. 4) Appliance according to one of claims 1 to 3, characterizedin that the diffraction grating (12) has curved grating lines (13). 5)Appliance according to claim 4, characterized in that one grating line(13′) is straight. 6) Appliance according to one of the precedingclaims, characterized in that the diffraction grating (12) is optimizedto first-order diffraction for both wavelengths (λ₁, λ₂). 7) Applianceaccording to one of the preceding claims, characterized in that thefirst laser diode (LD1) and the second laser diode (LD2) are arrangedtilted with respect to the optical axis (9). 8) Appliance according toone of the preceding claims, characterized in that the diffractiongrating (12) is arranged tilted with respect to the optical axis (9). 9)Appliance according to one of the preceding claims, characterized inthat the diffraction grating (12) is oriented such that side spots areoriented at right angles to information tracks on the optical recordingmedium (1). 10) Appliance according to one of the preceding claims,characterized in that the first laser diode (LD1), the second laserdiode (LD2) and the diffraction grating (12) are integrated in onemodule (14). 11) Appliance according to one of claims 1 to 6,characterized in that the diffraction grating (12) is arranged upstreamof the photodetector (8) in the beam path coming from the recordingmedium (1). 12) Appliance according to claim 11, characterized in thatthe diffraction grating (12) and the photodetector (8) are integrated inone module (14′). 13) Appliance according to one of the precedingclaims, characterized in that a further diffraction grating (15), inparticular a Ronchi grating, is arranged in the beam path. 14) Applianceaccording to one of the preceding claims, characterized in that bothlaser diodes (LD1, LD2) are operated simultaneously in order to recordinformation on the optical recording medium (1), while only one of thelaser diodes (LD1, LD2) is in each case operated for reading. 15) Methodfor producing a diffraction grating (12), in particular for use in anappliance according to one of the preceding claims, characterized inthat the grating structure and the grating line profile (h₁, h₂, h₃) aredefined, the corresponding height profile is subdivided into surfaces ofequal height, and the surfaces of equal height are transferred to ablank by means of lithography and an etching process.